<?php
    $servername = "localhost";
    $username = "samsci";
    $password = "scumpa98";
    $dbname = "my_samsci";
    $username_post = $_POST['username'];
    $password_post = $_POST['password'];

if (!empty($username_post) && !empty($password_post)) {

    $conn=mysql_connect($servername,$username,$password) or die ("Impossibile connettersi al server: " . mysql_error());

    mysql_select_db($dbname) or die ("Impossibile connettersi al database selezionato " . mysql_error());

    $query = "SELECT * FROM user WHERE username = '$username_post'
        AND password = '$password_post'";
    $result = mysql_query($query);

    if (mysql_num_rows($result) > 0) {

        echo "ciao benvenuto ". $username_post;

    } else {
        echo "Nome utente o password errati";
    }
} else {
    echo "Non hai compilato tutti i campi";
}